The invention relates generally to a device and method for measuring the specific density of a gaseous or liquid medium. More specifically, the invention relates to a device in which an acoustic transducer is provided and the transducer is excited to emit acoustic pulses and acoustic signals reflected through the medium to be measured and through a reference medium are compared to determine the desired characteristics of the medium to be measured.
German Patent Number DE 195 35 848 C1, which is incorporated herein by reference for all it teaches, discloses a device for measuring the acoustic impedance of liquid media using a multilayer acoustic transducer arrangement. An acoustic transducer simultaneously injects an acoustic pulse into a first and a second acoustic delay path, respectively. The acoustic transducer receives, and an evaluation device evaluates, the acoustic waves reflected at a boundary layer between the first delay path and a medium to be inspected as well as the acoustic waves reflected at a boundary layer between the second delay path and a reference medium. The ratio of the amplitudes of the two received acoustic pulses can be used to determine the acoustic impedance or the specific density of the measured medium. The two acoustic delay paths are essentially cylindrical in shape. A disk-shaped acoustic transducer of the same diameter is arranged between the two bases of the cylindrical delay paths.
One drawback of the prior art device described above is that so-called diffracted waves that are generated in the area of the edge of the disk-shaped acoustic transducer cause parasitic echoes in the delay paths, which are superimposed on the measurement signal and reduce the signal-to-noise ratio. In addition, the refracted waves excite surface waves on the bases of the delay paths, which also limit the accuracy of the measurement. Another drawback of the prior art device is that the acoustic transducer, which is embodied as a disk-shaped piezo-ceramic element, is subject not only to axial mode vibrations, which are desired for the measurement, but also the acoustic transducer freely vibrates in radial mode. These additional drawbacks further diminish the accuracy of the measurement.
To address the deficiencies in the prior art device described above, as well as other disadvantages of other prior art devices, an object of the present invention is to provide a device for measuring the specific density of a gaseous or liquid medium with improved measuring accuracy.
To attain this and other objects, a device used in the measurement of the specific density of a gaseous or liquid medium is proposed, the device having an acoustic transducer active on both of two sides and being operable to emit and receive acoustic signals. At least two substantially cylindrical acoustic delay paths, each having a respective known acoustic impedance, are provided, where one of the delay paths is arranged on one of the two sides of the acoustic transducer and on its base, facing away from the acoustic transducer, this delay path has a first boundary surface to the medium to be measured. Further, a second delay path is arranged on the other of the two sides of the acoustic transducer and on its base, facing away from the acoustic transducer, the second delay path has a second boundary surface to a reference medium whose characteristics are known. The acoustic transducer has a smaller diameter than the bases of the delay paths and the acoustic transducer is inserted into a substantially hollow cylindrical ring, an inside diameter of which is adapted to an outside diameter of the acoustic transducer and an outside diameter of which is adapted to the diameter of the bases of the delay paths.
In accordance with the present invention, due to a smaller diameter of the acoustic transducer compared to the diameter of cylindrical delay paths, diffracted waves produce fewer and weaker parasitic echoes in the delay paths, and the strength of the surface waves on the bases of the delay paths is reduced. A ring into which the acoustic transducer is inserted dampens the radial vibrations of the acoustic transducer. A further advantage according to the present embodiment results from the robust construction of the device, which is realized by the addition of the ring, since the forces acting between the delay paths can be absorbed by the ring and do not affect the boundary surfaces between the acoustic transducer and the delay paths.
The outside diameter of the acoustic transducer is preferably selected to be between one quarter and three quarters of the diameter of the bases of the delay paths. This dimensioning has the advantages that the acoustic transducer emits a substantially flat wave front into the delay paths and sufficient acoustic energy for the measurement is produced.
Since the acoustic transducer is embedded in the ring between the two delay paths in a positive fit, it is important that the thermal expansion coefficients of the employed materials be approximately equal. If the thermal expansion coefficients are approximately equal, thermal stresses created within the device under fluctuating temperatures are relatively low and the required long-term stability of the acoustic coupling of the components is achieved. It is, therefore, advantageous to embody the acoustic transducer as, for example, a piezo-ceramic element and to make the two delay paths, and the ring, of quartz glass or Zerodur.
The electrodes of the acoustic transducer can be produced by vacuum coating the bases of the delay paths facing the acoustic transducer. The vacuum coating technique has the advantage that the electrodes can be extended up to the lateral surfaces of the delay paths and can be readily contacted at that location.
According to another embodiment, an electrically conductive adhesive, particularly an electrically conductive epoxy resin adhesive, can be used to bond the bases of the delay paths facing the acoustic transducer. This eliminates the vacuum coating step of the delay paths, since the adhesive can perform the function of the electrodes of the acoustic transducer. In this embodiment as well, the electrodes can be easily extended up to the lateral surfaces, so that the device as a whole can be cost-effectively produced.
Another low-cost process for producing the electrodes involves the application of a metal foil to metallize the respective base.